1. Field of the Invention
The present invention relates to a technique for causing the flow of a switching current to an inductive load by use of semiconductor switching elements, and more particularly, to an inductive load driving method and an H-bridge circuit control device in which an H-bridge is formed by a plurality of semiconductor switching circuits and an inductive load, wherein a switching current flowing through the inductive load is controlled.
2. Description of the Related Art
In general, a stepping motor has a rotor including a rotatable moving magnet around which a plurality of driving coils composed of electromagnets are arranged. In the stepping motor, the position and speed of rotation of the rotor can be controlled in an open-loop manner by selecting one of the driving coils so that a current having a predetermined amplitude flows pulse-wise through the selected driving coil. Therefore, the stepping motors have been widely used as convenient motors.
As a method for driving such a stepping motor, there are widely used a unipolar driving method in which current flows in a fixed direction and a bipolar driving system in which a current flows in both of forward and reverse directions.
In either driving system, the amount of rotation (or step angle), which is a basic matter of the stepping motor, is determined by the number of arranged driving coils. In recent years, however, there has been employed a microstep driving technique in which in order to suppress vibration at the time of rotation or precisely control of angle the rotation, a constant current flowing through the driving coil is step-wise changed, as shown in FIG. 13A, so that the rotor is transiently stopped at an angle smaller than the basic step angle.
In the case where microstep driving is performed, a current detecting resistor is inserted in an H-bridge circuit so that the amplitude of a current supplied from a power source to a stepping motor is detected as a detection voltage which is output at the current detecting resistor. The detection voltage is compared with a predetermined reference voltage to cause the turn-on/off of the semiconductor switching elements so that a switching current 102 having a fixed amplitude, as shown in
FIG. 13B, is caused to flow through a driving coil, thereby stopping the rotor at an angle finer than a basic step angle.
Further, for example, in the case where the amplitude of the switching current flowing through the driving coil is to be changed from a current I.sub.2 to a current I.sub.3 smaller than the current I.sub.2, all the semiconductor switching elements are turned off to cause the flow of a regenerative current. Thereby, an energy stored in the driving coil is transferred to the power source so that the current flowing through the driving coil is rapidly decreased, as shown by a leftward descending waveform indicated by reference numeral 103.
In order to prevent a negative voltage from being inputted to a driving circuit for the H-bridge circuit, it is a general technique that the current detecting resistor inserted in the H-bridge circuit is connected such that a current regeneratively returned to the power source does not flow therethrough. Accordingly, it is not possible to detect the current flowing through the driving coil during a time when the switching current is decreased by the power source regeneration.
Therefore, in the prior art, the regenerative current flows only during an estimated time T until the switching current reaches a predetermined value. In that case, the current flowing through the stepping motor may decrease to a smaller value than is required, for example, as shown by a current amount indicated by reference numeral 104. Otherwise, before the current flowing to the stepping motor decreases to a desired current amount, the transfer to a steady operation for flowing a predetermined stepping current may be caused so that a current is supplied to the driving coil. This will cause the vibration of the stepping motor. Also, in the case where the power source regeneration based on such a time setting is performed, it is required that a proper time should be determined for each stepping motor to set a time for power source regeneration again since the value of an inductance and the value of an equivalent resistor change each time the kind of the stepping motor is changed. This is troublesome. A solution to this problem has been desired.
Further, when an energy stored in the driving coil is to be released, it is necessary to cause the flow of a current through a flywheel diode. However, the flywheel diode capable of being incorporated in a power IC having been popularized in recent years is a PN diode and has a long reverse recovery time (Trr). As a result, when the driving coil is connected to the power source after the termination of a period of time for release of the energy stored in the driving coil so that a current is supplied to the driving coil, a reverse through current will flow through the flywheel diode through which a forward current has flowed.
Also, if one of the semiconductor elements is turned on/off with one of the other semiconductor elements remaining in a turned-on a state in order that the switching current flowing through the driving diode is kept at a fixed amplitude, the potential of one end of the driving coil will change on a large scale. Since a stray capacitor equivalently connected in parallel to an inductance component of the driving coil exists in the stepping motor, such large change of the potential of the one end of the driving coil will cause a rush current for charging the stray capacitor.
In the case where such a through current or rush current flows through the current detecting resistor, there will be detected a large current which is not flowing through the inductance component of the driving coil. As a result, there is a problem that the power IC makes an erroneous detection operation.
In the prior art, a countermeasure such as the provision of a noise eliminating filter for the current detecting resistor is taken in order to prevent such an erroneous operation. However, this entails a problem that the cost becomes very high. Therefore, a solution of aft that problem has been desired.